This document contains the release notes for the LLVM compiler
infrastructure, release 2.2. Here we describe the status of LLVM, including
major improvements from the previous release and any known problems. All LLVM
releases may be downloaded from the LLVM
releases web site.

Note that if you are reading this file from a Subversion checkout or the
main LLVM web page, this document applies to the next release, not the
current one. To see the release notes for a specific releases, please see the
releases page.

This is the last LLVM release to support llvm-gcc 4.0, llvm-upgrade, and
llvmc in its current form. llvm-gcc 4.0 has been replaced with llvm-gcc 4.2.
llvm-upgrade is useful for upgrading llvm 1.9 files to llvm 2.x syntax, but you
can always use an old release to do this. llvmc is currently mostly useless in
llvm 2.2, and will be redesigned or removed in llvm 2.3.

LLVM 2.2 fully supports both the llvm-gcc 4.0 and llvm-gcc 4.2 front-ends (in
LLVM 2.1, llvm-gcc 4.2 was beta). Since LLVM 2.1, the llvm-gcc 4.2 front-end
has made leaps and bounds and is now at least as good as 4.0 in virtually every
area, and is better in several areas (for example, exception handling
correctness, support for Ada and Fortran, better ABI compatibility, etc). We
strongly recommend that you
migrate from llvm-gcc 4.0 to llvm-gcc 4.2 in this release cycle because
LLVM 2.2 is the last release that will support llvm-gcc 4.0: LLVM 2.3
will only support the llvm-gcc 4.2 front-end.

The clang project is an effort to build
a set of new 'llvm native' front-end technologies for the LLVM optimizer
and code generator. Currently, its C and Objective-C support is maturing
nicely, and it has advanced source-to-source analysis and transformation
capabilities. If you are interested in building source-level tools for C and
Objective-C (and eventually C++), you should take a look. However, note that
clang is not an official part of the LLVM 2.2 release. If you are interested in
this project, please see its web site.

A research team led by Scott Michel in the Computer Systems Research
Department at The Aerospace Corporation contributed the CellSPU backend, which
generates code for the vector coprocessors on the Sony/Toshiba/IBM Cell BE
processor. llvm-gcc 4.2 supports CellSPU as a 'configure' target and progress
is being made so that libgcc.a compiles cleanly. Notable pieces still in
development include full 64-bit integer and full double precision floating
point support.

Anton and Duncan significantly improved llvm-gcc 4.2 support for the GCC Ada
(GNAT) and Fortran (gfortran) front-ends. These front-ends should still be considered
experimental however: see the list of known problems.
The release binaries do not contain either front-end: they need to be built from
source (the Ada front-end only builds on x86-32 linux).

Dale contributed full support for long double on x86/x86-64 (where it is 80
bits) and on Darwin PPC/PPC64 (where it is 128 bits). In previous LLVM
releases, llvm-gcc silently mapped long double to double.

Gordon Henriksen rewrote most of the Accurate Garbage Collection code in the code generator, making the
generated code more efficient and adding support for the OCaml garbage collector
metadata format.

Christopher Lamb contributed support for multiple address spaces in LLVM
IR. This is useful for supporting targets that have 'near' vs 'far' pointers,
'RAM' vs 'ROM' pointers, or that have non-local memory that can be accessed with
special instructions.

LLVM now includes a new set of detailed tutorials, which explain how to implement a
language with LLVM and shows how to use several important APIs.

Gordon contributed support for C and OCaml Bindings for the basic LLVM IR
construction routines as well as several other auxiliary APIs.

Anton added readnone/readonly attributes for modeling function side effects.
Duncan hooked up GCC's pure/const attributes to them and enhanced mod/ref
analysis to use them.

Devang added LLVMFoldingBuilder, a version of LLVMBuilder that implicitly
simplifies the code as it is constructed.

Ted Kremenek added a framework for generic object serialization to bitcode
files. This support is only used by clang right now for ASTs but is extensible
and could be used for serializing arbitrary other data into bitcode files.

Duncan improved TargetData to distinguish between the size/alignment of a
type in a register, in memory according to the platform ABI, and in memory when
we have a choice.

Reid moved parameter attributes off of FunctionType and onto functions
and calls. This makes it much easier to add attributes to a function in a
transformation pass.

We put a significant amount of work into the code generator infrastructure,
which allows us to implement more aggressive algorithms and make it run
faster:

Owen refactored the existing LLVM dominator and loop information code to
allow it work on the machine code representation. He contributed support for
dominator and loop information on machine code and merged the code for forward
and backward dominator computation.

Dan added support for emitting debug information with .file and .loc
directives on platforms that support it, instead of emitting large tables in the .s
file.

Evan extended the DAG scheduler to model physical register dependencies
explicitly and have the BURR scheduler pick a correct schedule based on the
dependencies. This reduces our use of the 'flag' operand hack.

Evan added initial support for register coalescing of subregister
references.

Rafael Espindola implemented initial support for a new 'byval' attribute,
which allows more efficient by-value argument passing in the LLVM IR. Evan
finished support for it and enabled it in the X86 (32- and 64-bit) and C
backends.

The LLVM TargetInstrInfo class can now answer queries about the mod/ref and
side-effect behavior of MachineInstr's. This information is inferred
automatically by TableGen from .td files for all instructions with
patterns.

Evan implemented simple live interval splitting on basic block boundaries.
This allows the register allocator to be more successful at keeping values in
registers in some parts of a value's live range, even if they need to be spilled
in some other block.

The new MachineRegisterInfo.h class provides support for efficiently
iterating over all defs/uses of a register, and this information is
automatically kept up-to-date. This support is similar to the use_iterator in
the LLVM IR level.

The MachineInstr, MachineOperand and TargetInstrDesc classes are simpler,
more consistent, and better documented.

In addition to a huge array of bug fixes and minor performance tweaks, the
LLVM 2.2 optimizers support a few major enhancements:

Daniel Berlin and Curtis Dunham rewrote Andersen's alias analysis to be
several orders of magnitude faster, and implemented Offline Variable
Substitution and Lazy Cycle Detection. Note that Andersen's is not enabled in
llvm-gcc by default, but can be accessed through 'opt'.

Dan Gohman contributed several enhancements to Loop Strength Reduction (LSR)
to make it more aggressive with SSE intrinsics and when induction variables are
used by non-memory instructions.

Evan added support for simple exit value substitution to LSR.

Evan enhanced LSR to support induction variable reuse when the induction
variables have different widths.

Evan contributed support to the X86 backend to model the mod/ref behavior
of the EFLAGS register explicitly in all instructions. This gives more freedom
to the scheduler, and is a more explicit way to model the instructions.

Dale contributed support for exception handling on Darwin/PPC and he and
Anton got x86-64 working.

Evan turned on if-conversion by default for ARM, allowing LLVM to take
advantage of its predication features.

Bruno added PIC support to the MIPS backend, fixed many bugs and improved
support for architecture variants.

We rewrote the lexer and parser used by TableGen to make them simpler
and cleaner. This gives tblgen support for 'caret diagnostics'. The .ll file
lexer was also rewritten to support caret diagnostics but doesn't use this
support yet.

Dale has been grinding through the GCC testsuite, and marked many
LLVM-incompatible tests as not-to-be-run (for example, if they are grepping
through some GCC dump file that LLVM doesn't produce), he also found and fixed
many LLVM bugs exposed by the testsuite.

Intel and AMD machines running on Win32 with the Cygwin libraries (limited
support is available for native builds with Visual C++).

Sun UltraSPARC workstations running Solaris 8.

Alpha-based machines running Debian GNU/Linux.

Itanium-based machines running Linux and HP-UX.

The core LLVM infrastructure uses
GNU autoconf to adapt itself
to the machine and operating system on which it is built. However, minor
porting may be required to get LLVM to work on new platforms. We welcome your
portability patches and reports of successful builds or error messages.

This section contains all known problems with the LLVM system, listed by
component. As new problems are discovered, they will be added to these
sections. If you run into a problem, please check the LLVM bug database and submit a bug if
there isn't already one.

The following components of this LLVM release are either untested, known to
be broken or unreliable, or are in early development. These components should
not be relied on, and bugs should not be filed against them, but they may be
useful to some people. In particular, if you would like to work on one of these
components, please contact us on the LLVMdev list.

The -cee pass is known to be buggy and will be removed in
LLVM 2.3.

The MSIL, IA64, Alpha, and MIPS backends are experimental.

The LLC "-filetype=asm" (the default) is the only supported
value for this option.

C++ programs are likely to fail on IA64, as calls to setjmp are
made where the argument is not 16-byte aligned, as required on IA64. (Strictly
speaking this is not a bug in the IA64 back-end; it will also be encountered
when building C++ programs using the C back-end.)

The C++ front-end does not use IA64
ABI compliant layout of v-tables. In particular, it just stores function
pointers instead of function descriptors in the vtable. This bug prevents
mixing C++ code compiled with LLVM with C++ objects compiled by other C++
compilers.

There are a few ABI violations which will lead to problems when mixing LLVM
output with code built with other compilers, particularly for floating-point
programs.

llvm-gcc does not currently support Link-Time
Optimization on most platforms "out-of-the-box". Please inquire on the
llvmdev mailing list if you are interested.

Notes

llvm-gcc does not support __builtin_apply yet.
See Constructing Calls: Dispatching a call to another function.

llvm-gcc partially supports these GCC extensions:

Nested Functions:
As in Algol and Pascal, lexical scoping of functions.
Nested functions are supported, but llvm-gcc does not support
taking the address of a nested function (except on X86 targets)
or non-local gotos.

The llvm-gcc 4.2 Ada compiler works fairly well, however this is not a mature
technology and problems should be expected.

The Ada front-end currently only builds on x86-32. This is mainly due
to lack of trampoline support (pointers to nested functions) on other platforms,
however it also fails to build on x86-64
which does support trampolines.

A wide variety of additional information is available on the LLVM web page, in particular in the documentation section. The web page also
contains versions of the API documentation which is up-to-date with the
Subversion version of the source code.
You can access versions of these documents specific to this release by going
into the "llvm/doc/" directory in the LLVM tree.

If you have any questions or comments about LLVM, please feel free to contact
us via the mailing
lists.